A turning brake controlling system for applying braking force on the upper turning body of a power shovel is arranged to be actuated automatically to apply braking force on the upper turning body when the turning brake is actuated intentionally by the operator of the power shovel or when the turning lever for turning the upper turning body is shifted to its neutral position in order to stop the turning of the upper turning body. The turning brake means is usually adapted to be actuated several seconds after the turning lever is shifted from its "turning" position to its "neutral" position, thereby preventing sudden stoppage of the upper turning body.
Since the delay time for the turning brake; that is; the time which passes from the shifting of the turning lever from its "turning" position to its "neutral" position until the actuation of the turning brake controlling system is predetermined, in the case of turning operations of the shovel on a slope, if positioning of the upper turning body in the turning direction is made by turning it slowly, then the upper turning body is turned during the delay time for the turning brake under the influence of gravity and leakage of fluid from the hydraulic motor so that it is difficult to stop it towards a target.
Further, in case the above-mentioned conventional turning brake controlling system is controlled by means of a microprocessor, if the microprocessor fails to fulfill arithmetic function as programmed for some cause such as noise or static electricity, etc., then the binary ON-OFF control signal becomes unstable, or only either ON output or OFF output can be developed. As a result, the controlling function of the turning brake controlling system is completely lost thus creating a dangerous condition, so that the operator cannot help stopping the operation.